Potassium permeability dominates, giving about minus 70 millivolts
Expansion
Typical neuronal resting potential is about minus 70 mV.
Equilibrium potentials by the Nernst equation:
- Potassium: about minus 90 mV
- Sodium: about plus 60 mV
- Chloride: about minus 70 mV
At rest the membrane is roughly 50 to 100 times more permeable to potassium than to sodium, so the potential sits near the potassium equilibrium value, pulled slightly positive by the small sodium leak. The Goldman equation combines all ions weighted by permeability.
The sodium-potassium ATPase contributes in two ways: it maintains the concentration gradients, and being electrogenic (3 sodium out for 2 potassium in) it contributes a few millivolts directly.
Clinical consequence: hyperkalaemia raises the potassium equilibrium potential, so the cell depolarises, inactivating sodium channels and causing weakness and cardiac conduction failure. Hypokalaemia hyperpolarises, making cells harder to excite.